Laboratory Setup

The laboratory setup used to measure the rate of transmission is shown in Figure 9. Light incident from a 250 W quartz halogen light source traverses the scintillator crystal where the spectrometer then records and sends the data to the Spectra Suite™

software program for analysis [17]. Measurement of light transmitted and reflected were recorded and used to determine the percentage of light absorbed within the crystal.

Photon intensity equals the sum of photon transmission, reflection and absorption, as the following equation indicates [18]

o t r a

I   I I I

(3) where Io is the initial photon intensity, and It, Ir and Ia equal photon intensity transmitted, reflected and absorbed, respectively.

4 The index of refraction for the candidate set of crystals is high when compared to glass (n ~ 1.5), but is within the same general range as other solid inorganic scintillator crystals.

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A high rate of transmission is desirable in scintillators for two purposes. First, it ensures that emitted photons from the inelastic neutron scattering will not be absorbed or unintentionally reflected in the crystal. It also permits scintillated photons, now in the visible or near visible spectrum, to reach the intended photodetector device. Photon loss in either step reduces efficiency [13, p. 31].

Transmittance T is the fraction of incident light that passes through the crystal and is simply the ratio of transmitted to initial light intensity [18].

t o

T I

 I

(4) 2. Results

The results of the transmission measurements are shown in Figure 10. All crystals display a strong transmittance that exceeds 0.70, with LGSO:Ce, CWO and LuAG:Ce achieving 0.85 or greater.

Figure 10. Transmittance T for the candidate set of scintillators. All crystals exceed 70% light transmittance, with several achieving 85% or

higher.

For comparison, Figure 11 is provided from [19] to show the strong agreement in T for a representative collection of inorganic scintillators that includes BGO and PWO from this study. It is interesting to note that the solid black dots represent the theoretical limits of T based on multiple bouncing between two parallel end surfaces and no internal absorption [19]. The calculations used in [19] are

1 ( )

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It can be seen from Figure 11 that the difference between the theoretical and experimental T is small, and attributed to internal absorption. To further support this claim, Figure 12 shows the percentage of absorption measured in BGO from this study. A simple rearranging of terms from [18] yields the equation for photon absorption.

a o t r

I   I I I (7)

The absorption of light in BGO is representative of all the candidate crystals in this study, with peak absorption not exceeding ~ 5% and mostly averaging ~ 2–3%. This finding is in strong agreement with the results measured in [19].

Figure 12. Photon absorption as a function of wavelength for BGO in the visible and near visible spectrum.

Based on measurements recorded in this experiment, the set of scintillator crystals in this study achieved high transmittance and low absorption of visible and near visible light. On average, 80% of light traversed the crystals and less than 3% was absorbed. The remaining light loss is attributed to reflection at the air/crystal interface and is due to the relatively high index of refraction for the scintillator crystals. The findings in this

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In the simplest terms, cathodoluminescence (CL) is the emission of photons from a material under excitation by an electron beam [2]. The photons are emitted at characteristic wavelengths from the material when it is subjected to electron bombardment. The CL measurements were conducted with the use of a scanning electron microscope (SEM) [20]. The theory of CL is similar to that described in Chapter II involving the scintillation process insofar as the bombarding electrons (vice gamma photons) facilitate a transfer of energy to electrons in the crystalline lattice resulting in the production of valence holes and conduction band electrons [20]. The resulting e-h pair generation, recombination and photon emission process in CL is otherwise similar to scintillation.

There are several quantitative benefits associated with CL measurements for the characterization of heavy inorganic scintillators. CL provides a spectral response spanning the desired wavelength domain, which is used to identify peak photon emission wavelengths [21]. According to [14, p. 43], scintillators with photon emission in the visible or near visible spectrum are preferred for detection by a photocathode (PMT);

therefore, the wavelength domain for this study ranged from approximately 350–750 nm.

In addition to peak photon emission, CL also measures the intensity, or photon count, at a given wavelength. Intensity of photon emission is among the most important factors in the scintillation process [14]; a weak response signal indicates poor scintillation yield based on the relative light output defined as

ph where LR is the number of photons emitted by the scintillator per unit of absorbed energy (usually 1 MeV), Nph is the number of emitted photons and Einc is the energy attributed to the radiation source [13, p. 20].

From the perspective of quality control, CL can also aid in the identification of contaminates, known as admixtures, that are inadvertently incorporated during the crystal growth process. Admixtures can undermine the performance of scintillators through false positive readings, and are discussed in the results for LuAG:Ce. When used as a quality control and quality assurance parameter, CL is a valuable test to ensure that a homogeneous crystalline matrix exists.

1. Laboratory Setup

The laboratory setup used for the CL measurements is shown in Figure 13. The SEM is a JEOL model 840A and is operated in the spot mode, meaning the electron beam is held in a static position and incident normal to the sample. The excitation energy varied between 5–20 keV; however, lower energy proved optimal due to unwanted electrical charging encountered at higher energies. A probe current of 6×10^-11 A and magnification from 600× to 1300× was used. An Oxford CL system with a 0.25 m path length monochromator performed the spectroscopy. The detector is a thermoelectrically cooled photomultiplier with a response range from 300–900 nm.

Figure 13. JEOL 840A SEM with Oxford CL system laboratory setup.

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of the candidate crystals displayed peak photon emission in the visible spectrum, but due to discrepancies in crystal growth techniques, some variations in the accepted values exist in the literature. Furthermore, for the Ce activated crystals, [14] provides no information concerning Ce concentrations, an important consideration discussed in the following paragraphs.

Table 3. Experimentally determined peak wavelengths for the candidate set of crystals using CL at 300 K.

Although most of the candidate crystals display peak emission wavelengths close to the accepted values given in Table 3, LuAG:Ce clearly exhibits a red shift from the literature [21], as Figure 14 illustrates. According to [21], peak photon emission is achieved at 545 nm with a 0.03% Ce concentration. However, in [22], H. Li et al. suggest that varying Ce concentrations may contribute to wavelength shifts. In [22], the maximum intensity and peak wavelength (545 nm) are measured at a Ce concentration of 0.5%, with red shift occurring as Ce concentrations decrease. Applying this rational, LuAG:Ce in Figure 14 should be red shifted to some wavelength greater than 545 nm since the Ce concentration is only 0.03%. Figure 15 depicts this red shift for varying Ce concentrations of LuAG:Ce using X-ray radioluminescence, with Ce concentrations greater than 0.5% resulting in quenching [22]. To further support this claim, Dr. Oleg

Sidletskiy⁵ stated that the LuAG:Ce sample used in the present study may have a Ce concentration 1–2 orders of magnitude lower than anticipated [23]. This may explain the severe red shift in LuAG:Ce seen in this experiment, but further analysis is warranted.

Figure 14. Previous results for LuAG:Ce show peak photon emission at 545 nm with a Ce concentration of 0.03%. This is in stark contrast to the 630 nm red shift measured in the current study, from [21].

5Chief of Department for Crystal Growth Technology at the Institute for Scintillation Materials, NAS of Ukraine. His team was responsible for the fabrication of all scintillator crystals used in this study.

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Aside from the peak emission wavelength, the intensity of photon emission as a function of wavelength was calculated for the candidate set. As mentioned previously, the intensity of photon emission is a critical factor for scintillators. If photons emerge in the visible spectrum but are too weak or too broad, with no clearly identifiable peaks, efficiency and/or sensitivity will suffer. This can be due to the signal being lost in the background (too weak), or one that is generated across a wide wavelength band (too broad). Since the intensity of photon emission is proportional to the intensity of the incident energy, arbitrary units are generally assigned in the literature when presenting CL findings, as seen in Figures 15 and 16. However, since all of the crystals in this study were subjected to the same parameters, a comparison in photon intensity is certainly in order.⁶

With an incident electron energy of 5 keV, the photon intensity for the crystals ranged from approximately 30 photons/s at peak emission for ZWO to approximately 7500 photons/s for BGO. The tungstate based crystals as a group exhibited the lowest

6 The only variable encountered for the candidate set of crystals is the physical dimensions of the samples. Width of the crystal will influence the response, but for the purposes of this study it assumed to be negligible.

intensity while BGO and the Ce activated crystals displayed the highest. Figure 16 shows the intensity recorded for ZWO and PWO. The erratic and noisy spectrum is attributed to low light yield according to [23]. Although PWO has a low light yield, it has the fastest response time (6 ns) of the candidate set [14, p. 66]; four orders of magnitude faster than ZWO; three orders faster than CWO; two orders faster than BGO; and one order faster than LuAG:Ce and LGSO:Ce [14].

Figure 16. Intensity as a function of wavelength for ZWO (a) and PWO (b) at 5 keV electron beam incident energy. The noisy signal is attributed

to low light yield in these scintillators.

The results for CWO and BGO, shown in Figure 17, indicate a stronger signal and good peak formation. The increased signal intensity for CWO compared to the other tungstate crystals is partially explained in [14, pp. 54–55] due to a high conversion factor of 0.01 electrons per incident eV. The conversion factor for BGO is even higher at 0.045 electrons per eV. With a greater number of free or quasi-free electrons in the crystal lattice, the greater the potential for photon emission. However, it is worth noting that no single factor fully explains photon emission. Several competing processes occur in the crystal during periods of stimulation that affect performance, and although the tungstate crystals share a similar chemical composition, their crystalline structures are different. In fact, CWO has a monoclinic structure P21/c, and is more similar structurally to LGSO [23].

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Based on the previous discussion concerning extrinsic luminescent scintillators (see Role of Activators in Chapter II), activated crystals generally exhibit a strong luminescent signal due to the formation of transition sites within the bandgap region of the scintillator (Figure 8). The concentration of the activator plays a tremendous role not only in the peak emission wavelength, but also in the intensity of the resulting photons [21]. This is clearly seen in Figure 16 for LuAG:Ce, where a 0.2% increase concentration of Ce, up to the optimal concentration of 0.5%, resulted in nearly a 55% increase in intensity [22]. The impact of Ce concentration does not, however, appear to affect the intensity of all activated crystals in the same manner, as Figure 18 shows for LGSO:Ce [24]. The spectral response for LGSO:Ce in this study, shown in Figure 19 (a), is in strong agreement with the literature and Figure 18 [24], at a Ce concentration of 0.3%.

Figure 18. Previous study detailing the intensity as a function of wavelength for LGSO:Ce. The activator concentration does not affect the luminescent intensity as drastically as other activated scintillator

crystals, from [24].

Figure 19 (b) for LuAG:Ce shows the uncontrolled admixture of Eu³⁺ ions in the crystal [23]. CL is well suited for identifying contaminants that can lead to false positive responses. When the bright emission at ~ 625 nm is disregarded, the maximum intensity of approximately 3600 photons/s is less than that of LGSO:Ce and according to [23], may be due to the low Ce concentration.

The CL measurements in this study confirmed the peak emission wavelengths for the candidate set of crystals, and identified the red shift in LuAG:Ce. All crystals displayed peak emission within the visible spectrum. When comparing the luminescent intensity, the tungstate crystals suffered from low light output, even with CWO’s relatively high conversion factor. The activated crystals of LGSO and LuAG registered strong signals at around 4000 photons/s, but BGO performed best at nearly 7500 photons/s. The self-activating qualities of BGO and high conversion factor clearly contributed to the high rate of light output.

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C. GAMMA INDUCED SCINTILLATION

The culminating experiment in this study was designed to quantify the candidate crystals’ scintillation response when subjected to radiation. Although a fast neutron source was preferred in order to confirm the inelastic scattering process, one was not currently available for use. Instead, gamma radiation sources were used to simulate the gamma photons normally resulting from the inelastic scattering event.⁷

1. Laboratory Setup

The apparatus used to quantify the gamma induced scintillation is shown in Figure 20, and was part of a dark room setup intended to minimize stray photon interference. The photomultiplier tube (PMT) is a thermoelectrically cooled Hamamatsu H7421-40 with a spectral response from 300–720 nm. It features a GaAsP photocathode with a quantum efficiency of 40% at peak wavelength of 580 nm. The Hamamatsu software was programmed to record photon counts in 100 ms intervals during the 180 s run time, for a total of 1800 data points.

7 Testing with sources of fast neutrons is certainly necessary in future work. However, most

spontaneous fission isotopes emit gamma radiation in addition to neutrons. This experiment has merit from the standpoint that incident gamma radiation is present from the onset of spontaneous fission, as well as from fast neutron inelastic scattering.

Figure 20. Hamamatsu H7421-40 PMT as part of a dark room laboratory setup used to measure the gamma scintillation response for the

candidate set of crystals.

The source of radiation was provided using Co-60 and Ba-133 sources. Table 4 provides general information concerning these sources, to include current activity due to decay, and the average gamma energy based on the abundance of photons emitted. The deposited energy in Table 5 was determined to provide an approximation of the relative amount of energy delivered to each crystal based on isotope source, and should be considered preliminary in its reported values. Alternative calculations using air as the irradiated volume were considered, with the assumption that air would act as a surrogate for the relative energy deposition from the two sources. In both instances, Co-60 was found to deliver approximately 3.5 times more energy than Ba-133, yet stimulated less light output in the majority of the crystals. Only ZWO produced a higher scintillation when subjected to Co-60.

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Date of Manufacture Oct. 2006 Oct. 2007

Current Activity (μCi) 0.34 0.62

Table 5. The estimated amount of deposited energy in the crystals.⁸ Mass of Crystal (g) Delivered Energy

from Co-60 (MeV/s)

Based on Equation (8), [14] provides scintillation light yield values for common inorganic scintillators. These values were used to order the candidate set of crystals from lowest to highest expected light output: PWO, BGO, LuAG:Ce, CWO, ZWO, LGSO:Ce.⁹ Table 6 provides these relative light output values, as well as the expected light yield from each radiation source based on delivered energy from Co-60 and Ba-133.

8 Deposited energy was estimated by calculating the dose from the sources in units of Rads using the RadPro calculator (www.radprocalculator.com) and then converting this into absorbed energy in MeV for each of the crystals.

9 Although the ordering of crystals was generally observed, the numerical values for light yield were not. For each of the crystals, a normalized light output was determined by multiplying the given crystal’s relative light output value, given in [14], by the average gamma energy per disintegration from Table 4.

Results are shown in Table 6. It is recognized that not all of the gamma energy from a single disintegration would be deposited in the crystal, but this calculation provides a basis for relative comparisons between the crystal responses.